Embodiments of the present specification relate generally to magnetic sensors, and more particularly to methods and systems for improving noise rejection in magnetic sensors used in surgical navigation systems.
Generally, navigation systems are used to provide position and orientation information corresponding to subjects of interest. Accordingly, navigation systems find use in application areas such as aviation, industrial operations, security, gaming, animation, motion sensing, and/or medical applications. For example, during interventional procedures, a surgical navigation system may be used to assist in rapid and accurate positioning and/or orientation of surgical instruments, implants, or other medical devices in a patient's body.
Specifically, certain surgical navigation systems provide a representation of the medical device in relation to an anatomical region of interest via images generated by an associated imaging system such as an X-ray or ultrasound system. Typically, the generated images may be registered to an overall position and orientation of the patient and/or a target anatomy. Additionally, as the medical device is positioned with respect to the patient anatomy, the images generated by the imaging system may be continually updated to reflect location coordinates for the medical device that are determined using the navigation system. The continually updated images allow a medical practitioner to manipulate the medical device to a desired position and/or orientation in the patient's body.
Certain surgical navigation systems employ electromagnetic sensors to determine a position and/or orientation of the medical device. Particularly, in conventional surgical navigation systems, the electromagnetic sensors may be implemented with coils or microcoils that are attached to the medical device and are configured to generate and/or detect magnetic fields. The navigation system measures a response of the coils to the magnetic field, and in turn, determines a position of the medical device based on the measured response.
Generally, microcoils having large size and/or operating at high frequencies, for example of about several kHz, provide satisfactory tracking information. However, the microcoils exhibit poor signal-to-noise ratio (SNR) and reduced range at lower frequencies and/or volumes. Additionally, the microcoils are susceptible to magnetic field distortions that arise from eddy currents in nearby conducting objects, such as surgical implements or imaging systems. As tracking techniques using microcoil-based navigation systems rely on a stable magnetic field or a known magnetic field map, unpredictable disturbances resulting from movement of metallic objects in the magnetic field reduce accuracy of the tracking technique, often rendering the tracking technique inadequate. Moreover, these microcoils are generally expensive to manufacture.
Accordingly, certain surgical navigation systems offer use of compact and relatively inexpensive magnetic sensors such as Hall-effect sensors, coil sensors, or various magnetoresistive sensors for determining position and/or orientation information. Anisotropic magnetoresistance (AMR) sensors, in particular, can detect fields as low as about 10−9 Tesla, are extremely small, and are easy to fabricate. AMR sensors, thus, appear particularly suitable for use in navigation systems.
However, when used in surgical navigation systems, cables connecting the AMR sensors to the system electronics often pick up considerable “noise” or interfering signals resulting in erroneous position and/or orientation information. This noise is further amplified due to a high gain amplifier employed to boost the typically low AMR sensor output prior to digitization. As accurately determining position and orientation of the medical device is significant for appropriate administration of treatment and/or for avoiding injury to patient anatomy, the noise in the AMR sensor-based measurements limits use of the AMR sensors in conventional surgical navigation systems.